The question of whether probable-cause warrants issued by a judge are needed is an open one because the high court stopped short of answering it. The court ruled in January, 2012, that attaching the device amounted to a constitutionally protected search because it was a trespass on a private vehicle.
March 19, 2013
"The President Barack Obama administration is claiming that authorities do not need court warrants to affix GPS devices to vehicles to monitor their every move."
"The administration maintains that position despite the Supreme Court’s infamous decision last year that concluded that attaching the GPS devices amounted to search protected by the Constitution."